Aluminum-magnesium alloys are important technological alloys for marine applications. With magnesium concentrations of 3 to 6%, along with other alloying additions and appropriate thermomechanical processing, the alloys are high strength, light weight, resistant to seawater corrosion, and weldable. These characteristics make these alloys attractive for lightweight, high speed, fuel efficient ships, amphibious craft, and land vehicle armor.
These qualities make aluminum a particularly useful metal for marine vessels. An important class of aluminum alloys that are widely used in Navy and commercial ships are the 5000-series aluminum alloys, often referred to as “5000 aluminum.” These alloys contain magnesium to enhance their strength, where the magnesium forms a solid solution having a magnesium concentration of between 3 and 6% in the aluminum bulk.
However, over time, and particularly under prolonged in-service exposure to high temperatures, the magnesium in these alloys migrates to the grain boundaries in the material, where, as can be seen in the optical metallography shown in FIG. 1, it can combine with the aluminum to form second phase “precipitates (“beta particles”) with having an approximate stoichiometry of Al3Mg2 at the grain boundaries. This environmentally induced process, known as “sensitization,” significantly reduces the material's intergranular corrosion resistance, and leads to stress corrosion cracking of the alloy.
The degree of sensitization (“DOS”) is related to the density of beta particles present at the grain boundaries. A DOS near zero corresponds to a beta particle density of about 60% or less, while a DOS of 40 or more corresponds to a nearly 100% beta particle density at the grain boundaries. If the beta particle density on the grain boundaries exceeds about 60 to 65%, continuous networks of the particles may form, resulting in accelerated intergranular corrosion rates. It has been observed that if the DOS exceeds about 30, significant degradation of the corrosion fatigue and stress corrosion properties can occur, which rapidly gets worse with further increase of DOS.
Such sensitization affects a large class of Navy ships, including the DDG 963, CG, and FFG classes, which use 5000 series aluminum alloys in their deck plates and/or superstructures, as well potentially the Littoral Combat Ship (LCS), Joint High Speed Vessel (JHSV), and Joint Maritime Assault Connector (JMAC) that also will use this alloy of aluminum to achieve their performance. An example of sensitization-induced cracking on a Navy ship can be seen in FIG. 2, which shows a crack in the aluminum deckplate of a CG-47 Ticonderoga Class cruiser. The CG-47 class, which uses alloy 5456-H116 in their deck and superstructure plating, has experienced severe degradation from sensitization. As can be seen in FIG. 2, the crack is several millimeters wide and extends all the way through the 5-millimeter-thick deck plate. See R. Schwarting, G. Ebel, and T. J. Dorsch, “Manufacturing techniques and process challenged with CG47 class ship aluminum superstructures modernization and repairs,” Fleet Maintenance & Modernization Symposium 2001: Assessing Current & Future Maintenance Strategies, San Diego, 2011. If such cracking occurs, the only permanent remedy is to replace the parts, which is an expensive activity and can only be done with the ship out of service. Consequently, it is highly desirable to prevent cracking before it occurs.
Studies show that the sensitization of aluminum can be reversed by heating the aluminum to a temperature which both causes the beta phase particles to dissociate and causes the magnesium to dissolve back into the aluminum bulk. This process is known as “desensitization.” See L. Kramer, M. Phillippi, W. T. Tack, and C. Wong, “Locally Reversing Sensitization in 5xxx Aluminum Plate,” Journal of Materials Engineering and Performance (2012) 21:1025-1029.
As illustrated in the plots shown in FIG. 3, such desensitization occurs only over a limited temperature range. At temperatures below about 230° C., the aluminum remains sensitized, while at temperatures above about 345° C., aluminum will begin to anneal and soften (i.e. lose strength). Consequently, the temperature of the aluminum alloy during desensitization must be kept between about 230° C. and about 345° C. for sensitization to occur without loss of strength in the metal.
Dissolving the beta phase requires that the temperature be raised above the solvus temperature of the alloy, which depends upon exact alloy composition and temper condition. Generally, the solvus temperature for the 5000 series alloys that experience sensitization will be higher than that for a pure binary aluminum-magnesium alloy, see Y. Zuo and Y. A. Chang, “Thermodynamic Calculation of the Al—Mg Phase Diagram,” CALPHAD, Vol. 17, No. 2, pp. 161-174 (1993), and will increase with additional concentrations of other alloying elements. For example, a pure binary alloy of aluminum and magnesium at 4.5 percent magnesium (i.e., an alloy having the same magnesium concentration as alloy 5083) has an estimated solvus temperature of 230° C., while commercial alloy 5083, which has additional constituents, has an experimentally measured solvus value of 290° C. See Y. K. Yang and T. R. Allen, “Determination of the beta Solvus Temperature of the Aluminum Alloys 5083,” Metallurgical and Materials Transactions A—Physical Metallurgy and Materials Science, Vol. 44A, Issue 11, pp. 5226-5233 (2013). Commercial alloy 5456, which has a nominal magnesium concentration of 5.5 percent, should have a solvus temperature above the binary alloy value of about 260° C.; although the actual solvus has not been experimentally measured.
In addition, as noted above, desensitization should not be performed at temperatures high enough to anneal the alloy. Although such high temperatures will desensitize the alloy, they also will considerably soften the alloy, reducing its strength. Standard reference sources list 345° C. as the typical annealing temperature for 5000 series alloys including 5083 and 5456. See, e.g., Heat Treating of Aluminum Alloys, American Society for Metals Handbook, Vol. 4, ASM International, Materials Park, Ohio, pp. 841-879 (1991). Thus, the temperature needed to achieve desensitization without softening in marine service alloys will generally be within the broad range between 230° C. and 345° C., with specific, narrower temperature ranges for alloy compositions being determined empirically in each case.
Various methods to heat the aluminum to a temperature sufficient for desensitization while keeping the temperature within this critical range have been proposed.
In one method, a flexible ceramic pad heater is used to apply heat to the surface of the sensitized aluminum. See L. Kramer, et al., supra. In another method, friction-stir processing is used to heat and thereby desensitize the metal. See, e.g., A. P. Reynolds and J. Chrisfield, “Friction Stir Processing for Mitigation of Sensitization in 5XXX Series Aluminum Alloys,” Corrosion, Vol. 68, No. 10 (2012), pp. 913-921.
However, there are significant problems with these approaches. Both approaches require intimate contact with the aluminum, so their efficiency can be compromised by the presence of surface irregularities such as weld seams. In addition, the pad heater is a slow process and locally heats the entire structure. Large-scale heating of the structure is undesirable because it potentially increases sensitization levels in areas around the zone being treated, it introduces residual stresses in weld connections to the underlying framing which can result in local fatigue cracking, and it exposes the interior of the ship, including sensitive electronics and equipment, to potentially damaging temperatures. Finally, if it heats the aluminum above the anneal temperature of 345° C. as shown in FIG. 3, it compromises the strength of the material. The friction-stir process is somewhat faster than pad heating and has the potential advantage of preferentially heating a shallower layer, but it is still impractical because the deck plating on a ship cannot support the considerable mechanical forces required for such a process.
Neither these nor any other approach has so far been deployed in the fleet, and the sensitization and the resulting susceptibility of 5000 aluminum to corrosion and other damage, remains a significant issue.